1. Field of the Invention
The present invention concerns a method in order, given a detection of a physical property within a slice of a subject in a magnetic resonance system, to determine the position of this slice relative to a region moving relative to the slice, as well as a correspondingly designed magnetic resonance system.
2. Description of the Prior Art
Three-dimensional, high resolution real time temperature imaging by means of magnetic resonance tomography (also called MR temperature imaging) has in recent years developed into an essential aid for the monitoring and control of non-invasive or minimally invasive tumor therapy methods. The determination of the proton resonance frequency shift (PRF) of water protons given a temperature change is used as a standard method for the temperature determination from MR images; see for example W. Wlodarzcyk et al.; Phys. Med. Bio., 44, Pages 607-626, 1999.
Fast imaging methods (for example EPI, “echoplanar imaging”) with measurement times shorter than 200 ms per slice in connection with reference-free methods allow the temperature determination of even moving organs given a continuous measurement; see for example M. Zhu et al.; MRM, 60 (3); Pages 536-541, 2008 or W. A. Grissom et al.; 17th International Meeting of International Society for Magn. Resonance in Medicine (ISMRM), Abstract #2516.
Problems result given real time temperature imaging due to the movement of a region or an organ between two acquisitions. This movement leads to false temperature values in the PRF method for temperature presentation, and a reference-free method for temperature measurement does not enable a dose calculation (i.e. a calculation of with what amount of heat a tumor is to be treated). A simulation of the slices acquired with a magnetic resonance system with regard to the moving region or organ is frequently insufficient, requires knowledge of breathing/cardiac cycles and is complicated in its practical realization.
In order to be able to determine the temperature curve at a defined point in a moving region or organ, for example, a segmentation and/or a non-rigid registration can be conducted. Such methods are still in development, however, and for MR data with a low signal-to-noise ratio (as occur in MR real time temperature imaging) results cannot be generated with sufficient speed and with sufficient quality for a quantification of the temperature. Moreover, parameters of these methods that are to be set are typically organ-specific, which additionally hinders the application of this method.